1. Field of the Invention
The invention relates in general to a structure of semiconductor integrated circuits (ICs), and more particularly to a photodiode structure.
2. Description of the Related Art
A photodiode is a light-sensitive semiconductor device having a P-N junction that coverts light into an electrical signal (also known as a photo-detecting device). Due to the presence of an electric field at the P-N junction, electrons in the N-doped layer and holes in the P-doped layer cannot normally diffuse across the junction in the absence of light. However, when sufficient light falls on the P-N junction, electron-hole pairs are generated by energy from the light. These electrons and holes are able to diffuse towards the junction. Due to the presence of an electric field at the junction, electrons will separate out towards the N-side and holes will separate out towards the P-side of the junction and accumulate there. Therefore, a current is able to flow across the P-N junction. Ideally, a photodiode should remain in open-circuit condition in the dark until light is shone on the junction.
In general photodiode devices are used as imaging sensors in different types of equipment, for example, PC cameras and digital cameras. One major defect of a conventional photodiode is its relatively large junction leakage current. Junction leakage current often leads to the build-up of a large dark current in products that employ a large number of imaging sensors. Furthermore, the large dark current is capable of producing abnormal bright spots on an imaging screen.
FIGS. 1A-1C are schematic, cross-sectional views showing the process of forming a conventional photodiode.
In FIG. 1A, a substrate 100 having a pad oxide layer 102 thereon is provided. The pad oxide layer 102 is used as sacrificial layer to protect the substrate 100 form being destroyed during following steps. A patterned silicon nitride layer (Si.sub.3 N.sub.4) 104 is formed on the pad oxide layer 102. An ion implantation I.sub.1 is performed to form a P-well 106 in the substrate 100.
In FIG. 1B, the silicon nitride layer 104 is used as a mask in a local oxidation of silicon (LOCOS) operation. The LOCOS operation is carried out in an atmosphere comprising oxygen (O), such as water vapor or O.sub.2, to form a field oxide (FOX) layer 108 in the substrate 100. In other words, an insulating layer is formed surrounding a device region.
Since water vapor and oxygen cannot penetrate the silicon nitride layer 104 easily, the field oxide layer 108 does not form in regions covered by the silicon nitride layer 104. However, water vapor and oxygen still can diffuse horizontally into the substrate 100 at the edges of the silicon nitride layer 104. Bird's beaks 110 are formed at the edge of the field oxide layer 108.
In FIG. 1C, a wet etching is carried out to remove the silicon nitride layer 104 and the pad oxide layer 102 between the field oxide layer 108. An N.sup.+ -doped region 110 is formed in the P-well 106 by an ion implantation I.sub.2. The N.sup.+ means that the doped region has a heavy N-type dopant. The N.sup.+ -doped region 110 is annealed to drive the implanted N-type ions into the P-well 106. A P-N junction forms between the N.sup.+ -doped region 110 and the P-well 106, so that a photodiode device is thus completed.
However, in the conventional photodiode device described above, bird's beaks form on each side of the field oxide layer of the device structure. Since stress at the bird's beaks is higher and more crystal defects occur from the stress at the bird's beaks than at other portion of the device structure, a large junction leakage current is generated there. A unit of imaging equipment that employs a large number of photodiodes thus has a large leakage current and produces a large dark current that results in abnormal bright spots on an imaging screen.